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United States Patent |
5,201,184
|
Roth
|
April 13, 1993
|
Method and apparatus for precooling the helium tank of a cryostat
Abstract
In a method and an apparatus for the precooling of a helium tank of a
cryostat, in particular an optical cryostat with optical components in the
helium tank, or an NMR cryostat, or a medical NMR cryostat for magnet
resonance imaging, which accepts a superconducting magnet coil, a simple
and cost effective precooling is achieved in that liquid nitrogen is
directed out of a storage container under small overpressure via a conduit
through an opening in the helium tank of a cryostat into a heat exchanger
arranged in the helium tank and therefrom, by way of an opening, out of
the cryostat and, finally, into a collecting container, in particular,
into a nitrogen tank surrounding the helium tank.
Inventors:
|
Roth; Gerhard (Karlsruhe, DE)
|
Assignee:
|
Bruker Analytische Messtechnik GmbH (DE)
|
Appl. No.:
|
709098 |
Filed:
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May 29, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
62/47.1; 62/50.2; 62/51.1 |
Intern'l Class: |
F17C 005/02 |
Field of Search: |
62/47.1,50.2,51.1,55.5
|
References Cited
U.S. Patent Documents
1941304 | Dec., 1933 | Heylandt | 62/50.
|
2247850 | Jul., 1941 | Rayburn | 62/51.
|
2702460 | Feb., 1955 | Gaugler | 62/527.
|
3371499 | Mar., 1968 | Hagenbach et al. | 62/55.
|
3720072 | Mar., 1973 | Berta et al. | 62/47.
|
3769806 | Nov., 1973 | Boissin et al. | 62/55.
|
4223540 | Sep., 1980 | Longsworth | 62/51.
|
4510771 | Apr., 1985 | Matsuda et al. | 62/51.
|
Foreign Patent Documents |
2906060 | Mar., 1979 | DE.
| |
3724562 | Jul., 1987 | DE.
| |
Primary Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Hackler; Walter A.
Claims
I claim:
1. Method for precooling a helium tank of a cryostat comprising the steps
of:
feeding liquid nitrogen from a storage container under low overpressure via
a conduit through an opening to the helium tank of the cryostat and into a
heat exchanger disposed in the helium tank prior to filling of the helium
tank with liquid helium; and
thereafter feeding the liquid nitrogen, via an opening, out of the helium
tank and into a nitrogen collecting container surrounding the helium tank
before filling the helium tank with liquid helium.
2. Method according to claim 1 wherein the nitrogen is fed into and removed
from the helium tank via a single entrance opening.
3. Method according to claim 1, wherein the pressure in the nitrogen
storage container is utilized to transport the nitrogen.
4. Method according to claim 1 wherein the current flow velocity of the
nitrogen is selected in order that the helium tank exhibits a precooling
temperature of about 100 K when the nitrogen collecting container is just
filled up by the liquid nitrogen exiting from the heat exchanger.
5. A cryostat comprising:
a nitrogen tank;
a helium tank disposed within said nitrogen tank;
entrance opening means, in an upper side of the cryostat, for enabling
access to the nitrogen and helium tank; and
heat exchanger means, configured for connection to a nitrogen storage
container and to a collecting container, for precooling the helium tank by
passing nitrogen therethrough from the nitrogen storage container and into
the collecting container.
6. The cryostat according to claim 5 wherein the collecting container
comprises the nitrogen tank surrounding the helium tank of the cryostat.
7. The cryostat according to claim 5 further comprising inlet and exit
conduit means for introducing and removing liquid nitrogen to and from the
heat exchanger said inlet and exit conduit means being configured as
coaxial double pipes.
8. The cryostat according to claims 7 wherein the inlet and outlet conduit
means for the heat exchanger means comprises a pipe with a lengthwise
running intermediate wall, the lengthwise wall separating inlet and outlet
conduits from each other.
9. The cryostat according to claim 8 further comprising mesh insert means
disposed in said heat exchanger means for preventing nitrogen vapor
accumulation on the pipe wall.
10. The cryostat according to claim 7 wherein the inlet and outlet means is
configured for decoupling from the heat exchanger means and sized for
removal from the cryostat entrance opening means.
11. The cryostat according to claim 5 further comprising a superconductive
magnet coil disposed in said helium tank and wherein the heat exchanger
means is disposed above the magnet and is configured as partially
circular.
12. The cryostat according to claim 5 wherein the heat exchanger means is
disposed in the helium tank in a position where helium atmosphere located
in the helium tank convectively streams around all sides of the heat
exchanger means.
13. The cryostat according to claim 12 wherein the heat exchanger means is
configured for collapsing and removal from the helium tank through the
entrance opening means.
14. The cryostat according to claim 13 wherein the heat exchanger means
includes fan or umbrella-like cooling fins which can be folded out or
swivelled.
15. The cryostat according to claim 14 wherein the cooling fins are
configured as spring extendible.
16. The cryostat according to claim 14 wherein the cooling fins are
disposed at a position for providing heat exchanging surfaces to the
convecting helium gas stream.
17. The cryostat according to claim 5 wherein the heat exchanger means is
disposed in a spaced apart relationship with the walls of the helium tank.
Description
BACKGROUND OF THE INVENTION
The invention concerns a method and an apparatus for precooling the helium
tank of a cryostat, in particular, an optical cryostat with optical
components in the helium tank, an NMR cryostat, or a medical NMR cryostat
for magnetic resonance imaging which accepts a superconducting coil.
Cryostats which can accept a superconducting magnetic coil are, by way of
example, known in the art from the area of nuclear resonance spectroscopy
or magnetic resonance imaging (DE-OS 29 06 060, DE-PS 37 24 562). These
cryostats exhibit a plurality of containers or tanks which are nested
within each other and the innermost one contains a magnetic coil which,
during operation, is filled with liquid helium at a temperature of
approximately 4 K. An outer tank contains fluid nitrogen with a
temperature of approximately 77 K. Both tanks are vacuum isolated with
respect to each other and with respect to room temperature, whereby the
evacuated intermediate space contains radiation shields at intermediate
temperatures so that both the heat transport as well as the heat radiation
is minimized. Before the initial operation or following a maintenance or
repair on the magnetic coil or on the cryostat, said cryostat must be
cooled down to the operating temperature. Many prodecures are known in the
art by which the cryostat can be cooled down to the operating temperature.
One procedure proposes filling the nitrogen tank with fluid nitrogen and
the helium tank with fluid helium, and to cool down in this manner.
Towards this end, however, large quantities of fluid helium are necessary
since the magnetic coil and the inner space of the cryostat must be cooled
down with the fluid helium only. The liquid helium, despite its low
temperature of 4 K, exhibits only a small capacity for heat absorption
compared to that of liquid nitrogen at a temperature of 77 K. In
consequence, in addition to the large quantity of fluid helium used which,
in addition, is very expensive, there is a sizeable time expense due to
the small heat absorption capability. These disadvantages are,
furthermore, not justified by the requirement that the helium tank come
solely in contact with the helium cooling medium in order to avoid
impurities, in particular, when the tank contains elements with large heat
capacities, for example, a superconducting magnet coil.
Another method proposes venting the evacuated intermediate space between
the nitrogen and helium tanks by means of dry nitrogen gas so that the
helium tank can be cooled down to the temperature of 77 K via a heat
conduction. This procedure has, however, the disadvantage that the
nitrogen must then be removed from the evacuated intermediate space.
Since, in general, this can only be partially achieved, the remaining
nitrogen freezes onto the cooled helium tank when the cryostat is later
put into operation. Furthermore, there are possible vacuum valve
operational faults which present, in turn, safety risks, since there is
the danger of an explosion as a result of the overpressure coming from the
evaporating condensed nitrogen or air. Moreover, this indirect cooling by
way of the intermediate evacuated space between the nitrogen tank and the
helium tank is ineffective, time consuming, and in addition, in the event
of one single interconnected intermediate vacuum space between the
nitrogen tank and the helium tank on the one hand and the ambient
environment on the other hand, leads to an icing of the outer jacket of
the cryostat.
Another and the most common method is to initially fill the helium tank
with liquid nitrogen, whereby the tank is cooled down to a temperature of
77 K. In consequence of this cooling down, most of the total heat is
removed from the cryostat. However, the liquid nitrogen must then be
completely removed from the helium tank following the cooling down
procedure. Remaining quantities of nitrogen cause reduction in the helium
storage time, which, with NMR magnets, corresponds generally to one year,
and, in particular, remaining quantities of nitrogen on the magnet coil
cause a reduction in the operating safety of the magnet coil, that is to
say, an increased risk of a quench, that is to say, an unintentional
transition from the superconducting to the normally conducting state of
the magnet coil. Furthermore, with optical cryostats, there is the danger
that optical components such as windows, mirrors or other optical
elements, become coated with frozen nitrogen. If the nitrogen exhibits a
certain fraction of oxygen, that is to say is contaminated with oxygen,
then this paramagnetic component will be attracted to the magnet coil
during operation. Furthermore, there is the additional increased danger
that in consequence of the disassembly needed during the filling procedure
or the exchange of two liquid gases, that ambient air and, thereby,
moisture can condense, which, in turn, can lead to operational failures
and reduce the storage time of the helium. Furthermore, there is an
increased safety risk associated with operational error.
The positioning of an additional nitrogen tank on the outside of the helium
tank which coaxially surrounds the helium tank is also known in the art of
cryostats. In this arrangement the nitrogen tank can then be filled with
fluid nitrogen, whereby, the wall of the helium tank and thereby the
inside of the helium tank are gradually cooled down. This method exhibits,
however, the disadvantage that, within the helium tank, an insufficient
amount of convection of the helium gas within the helium tank is produced
due to the symmetric arrangement of the nitrogen tank around the helium
tank. This symmetric arrangement is, however, essential for an even weight
distribution of the tank in the cryostat. Furthermore, there is an
additional substantial disadvantage in that the entire volume taken in by
this nitrogen tank does not contribute to the cooling during the
subsequent steady state operation of the cryostat. Even if the entire
nitrogen were removed from the tank after precooling of the cryostat and
this tank then filled with helium, the helium would not contribute to the
cooling of the superconducting magnet, since, the helium in this tank is
confined and cannot contribute to the convection and heat transfer in the
actual helium tank of the cryostat.
One can therefore in general conclude that, with the cryostat configured in
this manner, the space within the cryostat is not optimally utilized.
Therefore, an object of the invention is to provide a method and/or an
apparatus for the precooling of a helium tank of a cryostat in which the
above mentioned disadvantages are avoided.
SUMMARY OF THE INVENTION
This object is achieved by means of a method in that liquid nitrogen is
directed out of a storage container under small pressure via a conduit
through an access opening of the helium tank of a cryostat into a heat
exchanger configured in the helium tank, then, via an access opening, out
of the cryostat and finally into a collecting container, in particular,
into the nitrogen tank surrounding the helium tank. The access openings in
the helium tank are in general configured as towers. For reasons of
simplicity, the terminology towers will be used in subsequent discussion.
In the method according to the invention, the helium tank of the cryostat,
which is filled with helium gas and under atmospheric pressure, is cooled
down with liquid nitrogen which, in turn, exhibits a temperature of
approximately 77 K. The liquid nitrogen is introduced via a conduit into
the helium tank and removes heat from the helium gas located in the helium
tank by means of a heat exchanger and directs that heat out of the helium
tank via the streaming nitrogen. Due to the large heat capacity of the
liquid nitrogen, large quantities of heat can be removed from the helium
tank using small amounts of nitrogen. Thereby, it is advantageous when the
heat exchanger exhibits a relatively small volume so that it is
arrangeable practically anywhere within the helium tank. Finally, this
method has the significant advantage that the inside of the helium tank
comes only in contact with helium and is, therefore, contaminated neither
by nitrogen nor by other gases. Since, with this method, only helium gas
is located in the helium tank and the nitrogen in the helium tank is
introduced within a closed conduit, it is not necessary to clean the
helium tank after it has been cooled to a temperature in the range of 100
K, rather, it can immediately thereafter be filled with liquid helium. In
order to cool the helium tank and the magnetic coil located within the
helium tank to a temperature of 4 K from a temperature of approximately
100 K it is then therefore necessary to use a substantially less amount of
liquid helium, since the majority of the total heat of the cryostat has
already been removed through the cooling of the liquid nitrogen flowing
through the heat exchanger to a temperature of about 100 K. With this
method the consumption of helium is likewise minimized. This method
according to the invention therefore offers the advantage of a cost
effective and operationally reliable precooling of the helium tank of the
cryostat. It combines the advantage of cooling from room temperature to a
temperature in the range of 100 K via nitrogen with the advantage that the
helium tank and thereby the magnetic coil located within the tank or
optical components only come in contact with helium and are not
contaminated.
It is preferred when the nitrogen is introduced and removed by way of the
same tower in the helium tank. This has the advantage that the method can
also be used with cryostats exhibiting a single tower. With cryostats
exhibiting a plurality of towers, this method offers the advantage that
the other towers can be used for other purposes.
It is preferred when the pressure in the nitrogen storage container is
utilized for the transport of the nitrogen. When the stop valve of the
nitrogen storage container is opened, the nitrogen streams out of the
storage container via the conduit through the heat exchanger and therefrom
into the other container connected following the heat exchanger.
The nitrogen tank is precooled by the cold nitrogen gas leaving the heat
exchanger. After a certain time liquid nitrogen, in general, a gas-fluid
mixture, also flows out of the heat exchanger and into the nitrogen tank
This tank is then filled. Optimally, the helium tank with all its
components then has a temperature of approximately 100 K when the nitrogen
tank is exactly filled up. Advantageously, the flow velocity of the
nitrogen is so chosen. Since the nitrogen is fed into the nitrogen tank
surrounding the helium tank of the cryostat, a further advantage results
in that this nitrogen tank, due to its reduced heat capacity compared to
that of the magnet coil in the helium tank, cools faster than the helium
tank and, thereby, impurities in the vacuum portion condense on the
nitrogen tank and not on the helium tank.
Preferably, the heat exchanger and its associated feed and exit conduits
are flushed of nitrogen after completion of the precooling of the helium
tank. This has the advantage that the feed and exit conduits can be
disconnected from the heat exchanger and removed from the cryostat without
contaminating the inner region of the helium tank with nitrogen. The heat
exchanger can remain in the inside of the helium tank, that is to say, it
must not be removed from the cryostat after precooling. When fluid helium
is introduced into the helium tank after precooling, said liquid helium
floods the heat exchanger, but the heat exchanger in the helium tank
presents insignificant interference since its volume is relatively small.
In a further method, the heat exchanger is removed from the helium tank
after precooling of the helium tank. This has the advantage that the heat
exchanger can be utilized to precool other cryostats, that is to say, this
heat exchanger is not lost, and the nitrogen precooling circuit remains
absolutely decoupled from the helium tank.
The above mentioned object is achieved in accordance with the invention by
means of the apparatus in that a heat exchanger arranged within the helium
tank of the cryostat exhibits a liquid nitrogen lead pipe and a plurality
of cooling fins. Since the nitrogen within the heat exchanger is guided
within a pipe and since this pipe is surrounded with a plurality of
cooling fins, one achieves the advantage that, with respect to the amount
of nitrogen located in the helium container during the precooling, the
cooling surface of the heat exchanger is extremely large. In this manner,
the heat exchanger is capable of removing large quantities of heat from
the helium gas and transfering said heat to the nitrogen.
In a preferred embodiment, the heat exchanger is connected to the nitrogen
storage container at one end and, at the other end, to a collecting
container. Hereby, it is preferred when the nitrogen tank surrounding the
helium tank of the cryostat serves as the collecting container. This has
the advantage that the nitrogen container is simultaneously filled while
the helium tank is being precooled or, conversely, when the nitrogen
container is being filled for the first time, the helium tank is being
simultaneously precooled.
It is preferred when the feed and exit conduits of the heat exchanger are
configured as double pipes, as coaxial, or as a pipe divided by means of a
lengthwise wall. In this double pipe the nitrogen can then be introduced
to and removed from the heat exchanger located in the helium tank by means
of a single tower. It is advantageous when the double pipe can be simply
positioned from the outside onto the heat exchanger in a sealable and
disconnectable manner. Furthermore, the effective free cross section of
the tower is thereby completely taken advantage of for the introduction
and removal of the nitrogen.
A strong convection of the helium gas within the helium tank is achieved in
that the heat exchanger is arranged above the magnet and, in particular,
is configured with a partial circular shape. In this configuration and
form of the heat exchanger one causes the cooled helium gas to fall down
across the heat exchanger and thereby along the magnet. In this manner the
cooled helium gas effectively removes heat from the magnet, sweeps upward
over the other side of the magnet before returning to the heat exchanger
in a warmed up state. By configuring the heat exchanger on only one side
of the magnet, a circulation of helium gas is driven within the cryostat,
which is directed downwards on the side of the heat exchanger and upwards
around the magnet on the opposite side.
It is preferred when the helium atmosphere located in the helium tank
convectively streams around all sides of the heat exchanger. This has the
advantage that the heat exchanger can be equipped with cooling fins on all
of its sides which then, together, are surrounded with helium gas and can
remove heat therefrom. In this manner, a relatively large heat exchange
surface is achieved without increasing the amount of nitrogen streaming
through the heat exchanger.
Preferably, the heat exchanger exhibits a separation from the wall of the
helium tank. In this embodiment, care is taken that the wall of the helium
tank does not negatively influence the convection.
An improvement envisions that the heat exchanger is configured as a
retrofitable element. In this way, existing cryostats can be equipped with
a heat exchanger so that, at the next occasion, these cryostats can be
cooled down by means of the precooling method according to the invention.
In order to increase the turbulence of the nitrogen streaming in the pipe
of the heat exchanger and, thereby, the heat transport from the inner
walls of the pipe onto the nitrogen, it is, advantageously, envisioned
that the heat exchanger pipe which guides the nitrogen exhibits a mesh
insert with good heat conductivity, for example, from copper, turbulence
insert, or the like. This mesh insert serves to prevent the nitrogen
vapour accumulation on the walls of the pipe from interfering with the
heat transport from the pipe wall.
It is advantageous when the heat exchanger, in particular when the coupling
between the feed and exit conduits and the heat exchanger, is heatable. In
this manner, the icing on the heat exchanger or the like can be removed.
In particular, with flexible heat exchangers, the flexibility can be
restored.
In a preferred embodiment it is provided for that the heat exchanger is so
configured that it is removable from the helium tank, in particular,
without having to decouple the feed and exit conduits. This has the
advantage that the heat exchanger can be utilized for a plurality of
precoolings of cryostats and that there is no dead space in the cryostat
itself. This allows a complete separation of the precooling circuit from
the helium tank.
This heat exchanger can, preferentially, be configured in such a way that
it exhibits fan like or umbrella like cooling fins which can be folded out
or swivelled. The folding out or swivelling of the cooling fins transpires
after the heat exchanger is introduced to the position in the cryostat
which has been provided for. After cool down the cooling fins are folded
together before removal of the heat exchanger so that the heat exchanger
can be pulled out through the tower.
In a heat exchanger with umbrella like fold-out cooling fins, the umbrella
tip forms the upper end and the fold-out ends of the cooling fins the
under end of the heat exchanger which, in this manner, with folded in
cooling fins, the heat exchanger can be introduced into the helium tank
through one of its towers and opened up between the wall of the helium
tank and the outer surrounding wall of the magnet in the region above the
magnet or above its middle position. It is advantageous if this
umbrella-like configuration of the heat exchanger can exhibit an opening
mechanism on its under end which is activated when the introduced heat
exchanger reaches the bottom of the helium tank, whereby the opening
mechanism disengages arresting devices so that, by way of example, the
umbrella opens by spring action. A removal of a heat exchanger configured
in this manner is possible in a simple fashion in that, during removal of
the heat exchanger from the cryostat, in consequence of the narrow opening
cross section of the tower, the umbrella folds or lays itself together. In
addition to increased surface area, this type of heat exchanger exhibits
the advantage that it can be used many times and that no dead space
remains in the helium tank.
Advantageously, the cooling fins are spring extendable. In this
configuration, no special measures are required to open up the cooling
fins after introducing the heat exchanger into the inside of the helium
container. When removing the heat exchanger, the cooling fins are pressed
together against the spring force or folded together so that the heat
exchanger can be pulled out through the relatively small opening of the
tower.
An increase in the heat transport from the helium gas to the cooling fins
is, advantageously, achieved in that the cooling fins are arranged at an
angle to the convecting helium gas stream, in particular, partially
interlaced. In this manner, the helium gas is deflected by the cooling
fins while streaming by the heat exchanger and, thereby, comes in intimate
contact with the cooling fins and is cooled down.
Advantageously, the heat exchanger exhibits a ratio of cooling surface to
incorporated nitrogen volume of 0,5 to 5 m.sup.2 /1, in particular 2
m.sup.2 /1. In this embodiment it is particularly clear that, with a lost
heat exchanger, that is to say with a heat exchanger which remains in the
cryostat after precooling of the helium tank, the dead volume is
relatively small, although the heat transfer surface of the heat exchanger
is large.
In a preferred embodiment the cooling surface is 0,2 m.sup.2 and the volume
of the nitrogen in the heat exchanger 0,1 1.
After precooling, the liquid nitrogen which possibly still remains in the
heat exchanger is, through flushing with helium gas, removed in a manner
which is much more reliable than might possibly have been effected in the
separate large volume tank utilized by prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
Further characteristic features, advantages and details of the invention
can be derived from the following description in which preferred
embodiments are described in detail in reference to the drawings. Clearly,
the invention is not limited to the embodiment and the characterized
features which have been mentioned can be utilized by themselves and in
any combination without departing from the framework of the invention.
Shown in the drawings are:
FIG. 1 A sketch of principal operation of the cryostat which is to be
precooled;
FIG. 2 A first embodiment of the heat exchanger; and
FIG. 3 A second embodiment of the heat exchanger in a perspective view.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the sketch of the operation principal shown in FIG. 1, a cryostat
labelled with 2 and a nitrogen storage container 3 are connected to each
other via a heat insulated pipe conduit 1. A stop element labelled 4 with
a pressure reducer is located immediately connected to the nitrogen
container 3 which, by way of example, is configured as a pressure vessel,
said stop element being precisely regulatable with regard to the amount of
liquid nitrogen streaming out of the nitrogen storage container 3. The
cryostat 2 exhibits, within an outer jacket 5, as viewed from the outside
towards the inside, a nitrogen tank 6, a radiation shield 7, a helium tank
8, and a coil body 9 located within the helium tank 8 which, in
particular, is a superconducting magnet coil. The cryostat is equipped
with entrance openings 11 on the upper side 10 in which the individual
tanks 6 and 7 as well as shields, for example the radiation shield 7, are
suspended. Furthermore a filling support 12 with which the nitrogen tank 6
can be filled is located on the upper side 10. Entrance opening 11
configured as tower 11a exhibits an opening 13 which penetrates through
the outer jacket 5, a nitrogen radiation shield 14, and the radiation
shield 7 and leads into the helium tank 8.
Another pipe conduit 15 can be seen in FIG. 1 which leads out of the tower
11i a and is connected to the filling supports 12. The two pipe conduits 1
and 15 penetrate through the tower 11a by way of the through opening 13
and project into the helium tank 8 where they are connected to a heat
exchanger 16 arranged within helium tank 8. In the schematic
representation of FIG. 1, the heat exchanger 16 is not presented to scale,
rather, is pictured with exaggerated dimensioning so that the principal of
the invention is more clearly represented. This heat exchanger 16 is
represented in two preferred embodiments in the FIGS. 2 and 3 and is
further explained below. The two pipe conduits 1 and 15 exhibit a coupling
17 in proximity to the heat exchanger 16 by means of which said pipes are
connectable to the heat exchanger 16. In this manner the pipe conduit 1
constitutes the feed conduit for the heat exchanger 16 and the pipe
conduit 15 the exit conduit, whereby the two conduits 1 and 15 must not
necessarily be fed through the same tower 11a into the innards of the
helium tank 8. The tower 11a is connected to a helium storage vessel 22
via a conduit and a safety valve. The helium tank 8 always has a small
overpressure, in particular, during the phase where it is temporarily
opened to the outside. During the precooling phase, the helium gas streams
without hindrance from the storage vessel 22 into the helium tank 8.
The embodiment of a heat exchanger 16 represented in FIG. 2 is configured
as a plate heat exchanger and exhibits a cooling hose pipe 18 which
penetrates through ray-like or radially arranged cooling fins or cooling
plates 19. In this embodiment, the inlet and the outlet of the heat
exchanger 16 are provided for at the ends of the heat exchanger 16,
whereby in other embodiment types the one end could be the inlet and the
other end the outlet. In this embodiment, the plates 19 are penetrated
only once by the cooling hose pipe 18. The number of cooling fins 19 Can
preferentially be chosen in such a way that the heat exchanger 16 exhibits
the necessary heat exchanging surface. The heat exchanger 16 represented
in FIG. 2 exhibits largely a half circular form and is accordingly
arranged above the coil body 9 on one side of this coil body. This has the
substantial advantage that the helium atmosphere cooled by the heat
exchanger 16 in the region of the heat exchanger descends downwards along
the outer circumference surface of the coil body and thereby is able to
remove heat from the coil body 9. Correspondingly, the warm helium
atmosphere located on the opposite side of the coil body 9 rises upwards
and reaches into the region of the heat exchanger 16 which, in turn, is
able to remove the heat of the coil body 9 taken up by the helium. The
convection excited in this manner effects and even cooling of the coil
body 9 which, due to this careful handling, does not result in any
stresses from large temperature gradients which unavoidably occurs in
direct precooling of the helium tank 8 using liquid nitrogen.
The heat exchanger 16 represented in FIG. 3 concerns a movable heat
exchanger which can be introduced into the helium tank 8 via the opening
13 in the tower 11a or the tower 11b and, following the precooling
procedure, is removable from the cryostat 2. Accordingly, this heat
exchanger 16 exhibits a maximum diameter which is so dimensioned that it
can be passed through the opening 13 of the tower 11a or 11b. In this
manner, the feed conduit 20 and the exit conduit 21 form the core of the
heat exchanger 16, whereby the exit conduit 21 is arranged within the feed
conduit 20 or vice versa, that is to say, the core is configured as a
double pipe. Ray like or radially arranged cooling fins 19 are attached to
the outer wall of the inlet conduit 20, whereby the total number of
cooling fins 19 determines the entire heat exchanging surface. A heat
exchanger configured in this manner exhibits the advantage that it can be
used a plurality of times and that, by way of example, more than one heat
exchanger 16 can be instituted for the precooling of the cryostat 2. This
heat exchanger 16 has the substantial advantage that the nitrogen
circulation is absolutely sealed and closed in that no coupling connection
for inlet and exit conduits is necessary.
The precooling procedure for the cryostat is explained below. After
attaching the nitrogen container 3 via the pipe conduit 1 to the heat
exchanger 16 and connecting the heat exchanger 16 via the pipe conduit 15
to the filling support 12 for the nitrogen tank 6, the stop element is
opened and the pressure reducer 4 is adjusted in such a manner that the
nitrogen from the storage container 3 streams with low velocity via the
pipe conduit 1 into the heat exchanger 16. In this manner, the cooling
fins 19 of the heat exchanger 16 are cooled via the cooling hose pipe 18
which, in turn, as described above, cools the helium atmosphere located in
the helium tank 8. The warmed nitrogen then, via the pipe conduit 15,
streams into and flows through the nitrogen container 6 of the cryostat 2.
In this manner the nitrogen tank 6 is cooled, in an advantageous fashion,
more quickly than the helium tank 8, since the mass located in the helium
tank 8 of the coil body 9 cools substantially slower than the mass of the
nitrogen tank 6.
As has already been described above, a convection stream of helium
atmosphere in helium tank 8 is produced by means of which the cooling body
9 is carefully and evenly cooled down. At the beginning of the precooling
procedure liquid nitrogen streams into and cools the helium tank 8.
Thereby, the nitrogen vaporizes and exits the helium tank via the outlet
conduit of the heat exchanger 16 in a gaseous state. The nitrogen gas then
streams into and cools the nitrogen tank 6. Through the gradual cooling
down of the helium tank 8 the nitrogen no longer completely vaporizes so
that a gas-liquid mixture exits from the heat exchanger 16. The flow
velocity of the nitrogen is thereby so chosen that, when the nitrogen tank
6 is full, the helium tank 8 is precooled to a temperature of
approximately 100 K.
When the precooling procedure is ended, the two pipe conduits 1 and 15 are
separated from the nitrogen storage container 3 and from the fill supports
12, and the nitrogen located in the pipe conduits 1 and 15 and in the heat
exchanger 16 is flushed out via a flushing out procedure. Finally, helium
gas is introduced into the pipe conduits 1 and 15 and into the heat
exchanger 16. In this manner, the inlet and exit conduits which penetrate
the towers 11a or 11b through the opening 13 are separated from the heat
exchanger 16 and the coupling 17 and are removed from the cryostat 2.
Thereby one should emphasize that the helium tank 8 is under slight
overpressure, that is to say, there is no danger that ambient air can
penetrate in. The cryostat 2 is then ready for the helium tank 8 to be
filled up with liquid helium. Thereby, the heat exchanger 16 is also
filled with helium which, however, is not disadvantageous since, due to
the small volume of the cooling hose pipe 18 and the cooling fins 19, the
dead volume formed by the heat exchanger 16 within the helium tank 8 is
negligibly small.
With a heat exchanger 16 which is removable from the helium tank 8, said
heat exchanger 16 is removed from the cryostat 2 after the precooling
procedure and the tank 8 can immediately be filled with liquid helium. It
may be necessary to fold in extended the umbrellalike or fan like cooling
fins prior to removal of the heat exchanger 16.
The reference numerals in the patent claims are not a restriction but shall
facilitate understanding.
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